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The widespread of mixtures formed by polyelectrolytes and oppositely charged surfactants in different industrial sectors requires a thorough understanding of the physicochemical bases governing their interactions. These interactions lead to the formation of polyelectrolyte–surfactant aggregates, with the presence of such aggregates in the aqueous mixtures impacting decisively on their bulk and interfacial properties. Furthermore, the broad range of morphologies, sizes, and properties of the aggregates makes it necessary to obtain a comprehensive picture describing their behavior and properties to be able to exploit their possible applications. This review is aimed to shed light on some of the most relevant physicochemical aspects involved in the complexation process of polyelectrolyte–surfactant mixtures, paying special attention to the formation of nonequilibrium states (kinetically trapped aggregates), and how these impact on the interaction of the complexes with interfaces, both fluid/fluid and solid/fluid.

Particle-laden fluid/fluid interfaces are ubiquitous in academia and industry, which has fostered extensive research efforts trying to disentangle the physico-chemical bases underlying the trapping of particles to fluid/fluid interfaces as well as the properties of the obtained layers. The understanding of such aspects is essential for exploiting the ability of particles on the stabilization of fluid/fluid interface for the fabrication of novel interface-dominated devices, ranging from traditional Pickering emulsions to more advanced reconfigurable devices. This review tries to provide a general perspective of the physico-chemical aspects associated with the stabilization of interfaces by colloidal particles, mainly chemical isotropic spherical colloids. Furthermore, some aspects related to the exploitation of particle-laden fluid/fluid interfaces on the stabilization of emulsions and foams will be also highlighted. It is expected that this review can be used for researchers and technologist as an initial approach to the study of particle-laden fluid layers.

International audience; The formation of coffee-ring deposits upon evaporation of sessile droplets containing mixtures of poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants were studied. This process is driven by the Marangoni stresses resulting from the formation of surface-active polyelectrolyte–surfactant complexes in solution and the salt arising from the release of counterions. The morphologies of the deposits appear to be dependent on the surfactant concentration, independent of their chemical nature, and consist of a peripheral coffee ring composed of PDADMAC and PDADMAC–surfactant complexes, and a secondary region of dendrite-like structures of pure NaCl at the interior of the residue formed at the end of the evaporation. This is compatible with a hydrodynamic flow associated with the Marangoni stress from the apex of the drop to the three-phase contact line for those cases in which the concentration of the complexes dominates the surface tension, whereas it is reversed when most of the PDADMAC and the complexes have been deposited at the rim and the bulk contains mainly salt.

This work presents a simple methodology for coating small unilamellar liposomes bearing different degrees of positive charge with polyelectrolyte multilayers using the sequential layer-by-layer deposition method. The liposomes were made of mixtures of 1,2-dioleyl-sn-glycero-3-phosphocoline and dimethyl dioctadecyl ammonium bromide (DODAB) and coated by alternated layers of the sodium salt of poly(4-styrenesulfonate) (PSS) and poly(allylamine) (PAH) as polyanions and polycations, respectively. The results show that the zeta potential of the liposomes was not very sensitive to the mole fraction of DODAB in the membrane, XD, in the range 0.3 ≤ XD ≤ 0.8. We were able to coat the liposomes with up to four polymer bilayers. The growth of the capsule size was followed by dynamic light scattering, and in some cases, by cryo-transmission electron microscopy, with good agreement between both techniques. The thickness of the layers, measured from the hydrodynamic radius of the coated liposome, depends on the polyelectrolyte used, so that the PSS layers adopt a much more packaged conformation than the PAH layers. An interesting finding is that the PSS amount needed to reach the isoelectric point of the capsules increases linearly with the charge density of the bare liposomes, whereas the amount of PAH does not depend on it. As expected, the preparation of the multilayers has to be done in such a way that when the system is close to the isoelectric point, the capsules do not aggregate. For this, we dropped the polyelectrolyte solution quickly, stirred it fast, and used dilute liposome suspensions. The method is very flexible and not limited to liposomes or polyelectrolyte multilayers; also, coatings containing charged nanoparticles can be easily made. Once the liposomes have been coated, lipids can be easily eliminated, giving rise to polyelectrolyte nanocapsules (polyelectrosomes) with potential applications as drug delivery platforms.

The Layer-by-Layer (LbL) method is a well-established method for the assembly of nanomaterials with controlled structure and functionality through the alternate deposition onto a template of two mutual interacting molecules, e.g., polyelectrolytes bearing opposite charge. The current development of this methodology has allowed the fabrication of a broad range of systems by assembling different types of molecules onto substrates with different chemical nature, size, or shape, resulting in numerous applications for LbL systems. In particular, the use of soft colloidal nanosurfaces, including nanogels, vesicles, liposomes, micelles, and emulsion droplets as a template for the assembly of LbL materials has undergone a significant growth in recent years due to their potential impact on the design of platforms for the encapsulation and controlled release of active molecules. This review proposes an analysis of some of the current trends on the fabrication of LbL materials using soft colloidal nanosurfaces, including liposomes, emulsion droplets, or even cells, as templates. Furthermore, some fundamental aspects related to deposition methodologies commonly used for fabricating LbL materials on colloidal templates together with the most fundamental physicochemical aspects involved in the assembly of LbL materials will also be discussed.

The adsorption of mixtures of charged polymers onto solid surfaces presents a big interest in different technological and industrial fields, and in particular, in cosmetics. This requires to deepen on the most fundamental physico-chemical bases governing the deposition, which is generally correlated to the interactions occurring in solution. This work explores the interaction in solution of model polymer mixtures formed by a cationic homopolymer (poly (diallyl-dimethyl-ammonium chloride), PDADMAC) and a zwitterionic copolymer (copolymer of acrylic acid, 3-Trimethylammonium propyl methacrylamide chloride and acrylamide, Merquat 2003), and the adsorption of such mixtures onto negatively charged surfaces. The analysis of the interactions occurring in solution between both polymers performed using dynamic light scattering (DLS), electrophoretic mobility and viscosity measurements, combined with the study of the deposition of the layers of mixtures containing different weight fractions of each polymer using ellipsometry and quartz crystal microbalance with dissipation monitoring (QCM-D) has shown that the interpolymer complexes formed in solution, and their composition, governs the deposition onto the solid surface and the tribological properties of the adsorbed layers as shown the Surface Force Apparatus (SFA) experiments, allowing for a control of the physico-chemical properties and structure of the layers. Furthermore, the use of Self Consistent Mean Fields Calculations (SCF) confirms the picture obtained from the experimental studies of the adsorbed layers, providing a prediction of the distribution of the polymer chains within the adsorbed layers. It is expected that this study can help on the understanding of the correlations existing between the behavior of future associations of innovative and eco-sustainable polymers and their adsorption processes onto solid surface.

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

The understanding of the deposition of oppositely charged polyelectrolytes-surfactant mixtures onto solid surfaces presents a high interest in current days due to the recognized impact of the obtained layers on different industrial sectors and the performance of several consumer products (e.g. formulations of shampoos and hair conditioners). This results from the broad range of structures and properties that can present the mixed layers, which in most of the cases mirror the association process occurring between the polyelectrolyte chains and the oppositely charged surfactants in the bulk. Therefore, the understanding of the adsorption processes and characteristics of the adsorbed layers can be only attained from a careful examination of the self-assembly processes occurring in the solution. This review aims to contribute to the understanding of the interaction of polyelectrolyte-surfactant mixtures with solid surfaces, which is probably one of the most underexplored aspects of these type of systems. For this purpose, a comprehensive discussion on the correlations between the aggregates formed in the solutions and the deposition of the obtained complexes upon such association onto solid surfaces will be presented. This makes it necessary to take a closer look to the most important forces driving such processes.

Most of the currently used products for repairing and conditioning hair rely on the deposition of complex formulations, based on mixtures involving macromolecules and surfactants, onto the surface of hair fibers. This leads to the partial covering of the damaged areas appearing in the outermost region of capillary fibers, which enables the decrease of the friction between fibers, improving their manageability and hydration. The optimization of shampoo and conditioner formulations necessitates a careful examination of the different physicochemical parameters related to the conditioning mechanism, e.g., the thickness of the deposits, its water content, topography or frictional properties. This review discusses different physicochemical aspects which impact the understanding of the most fundamental bases of the conditioning process.

Carbonaceous particles are accounted among the most common environmental pollutants. This makes necessary a careful examination of their potential impact on the physico-chemical properties of different biological films, e.g. tear film, skin, or lung surfactant. This work analyzes the interaction of three different types of carbonaceous particles with Langmuir monolayers of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) spread at the water/vapor interface. This type of studies can be exploited as preliminary in vitro tests for evaluating the potential risks and hazards of this type of pollutants for human health, which requires to study the effect of the particles on the ability of lipid layers for reducing the surface tension under static and dynamics conditions. The obtained results have shown that the insertion of particles into lipid films leads to the emergence of excluded area-like effects which modify the interfacial cohesion and packing. This hinders partially the ability of the lipid films for reducing the surface tension, which may present important adverse effects for the normal physiological performance of lipid membranes, and in particular for the performance of lung surfactant layers. Furthermore, the analysis of the response of lipid layers including carbonaceous particles against mechanical stresses also evidenced that particle incorporation worsens the mechanical performance of lipid layers, which may alter both their functional and structural roles.

For many years the determination of the shear viscosity of interfacial layers has been source of strong controversy. This is mainly because different techniques provided different values of such parameter, which leads in many cases to a puzzling interpretation of the experimental results. One possible explanation of this is the non-correct analysis of the hydrodynamic conditions of the measurement and, in particular, the assumption of some approximations that may not be necessarily valid in all cases. The introduction of hydrodynamic consideration in the study of interfacial shear rheology has helped to clarify some of the existing discrepancies between measurements performed using different devices, thus allowing one to establish clearly the viscosity range in which different techniques can operate with enough sensitivity for determining the interfacial shear viscosity. This review puts in perspective the most recent developments on the studies of the interfacial shear rheology of fluid/fluid interfaces, analyzing the strength and weakness of the different approaches.

This work analyzes the dispersion of two highly hydrophobic actives, (9Z)-N-(1,3-dihydroxyoctadecan-2-yl)octadec-9-enamide (ceramidelike molecule) and 2,6-diamino-4-(piperidin-1-yl)pyrimidine 1-oxide (minoxidil), using oil-in-water nanoemulsions with the aim of preparing stable and safe aqueous-based formulations that can be exploited for enhancing the penetration of active compounds through cosmetic substrates. Stable nanoemulsions with a droplet size in the nanometric range (around 200 nm) and a negative surface charge were prepared. It was possible to prepare formulations containing up to 2 w/w% of ceramide-like molecules and more than 10 w/w% of minoxidil incorporated within the oil droplets. This emulsions evidenced a good long-term stability, without any apparent modification for several weeks. Despite the fact that this work is limited to optimize the incorporation of the actives within the nanoemulsion-like formulations, it demonstrated that nanoemulsions should be considered as a very promising tool for enhancing the distribution and availability of hydrophobic molecules with technological interest.

The assembly in solution of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants, sodium lauryl ether sulfate (SLES) and sodium N-lauroyl-N-methyltaurate (SLMT), has been studied. Additionally, the adsorption of the formed complexes at the water&ndash, vapor interface have been measured to try to shed light on the complex physico-chemical behavior of these systems under conditions close to that used in commercial products. The results show that, independently of the type of surfactant, polyelectrolyte-surfactant interactions lead to the formation of kinetically trapped aggregates in solution. Such aggregates drive the solution to phase separation, even though the complexes should remain undercharged along the whole range of explored compositions. Despite the similarities in the bulk behavior, the equilibration of the interfacial layers formed upon adsorption of kinetically trapped aggregates at the water&ndash, vapor interface follows different mechanisms. This was pointed out by surface tension and interfacial dilational rheology measurements, which showed different equilibration mechanisms of the interfacial layer depending on the nature of the surfactant: (i) formation layers with intact aggregates in the PDADMAC-SLMT system, and (ii) dissociation and spreading of kinetically trapped aggregates after their incorporation at the fluid interface for the PDADMAC-SLES one. This evidences the critical impact of the chemical nature of the surfactant in the interfacial properties of these systems. It is expected that this work may contribute to the understanding of the complex interactions involved in this type of system to exploit its behavior for technological purposes.

Provided here is a succinct survey of significant features of theory and experiments dealing with drops or bubbles which may act as traveling reactors or mere carriers of appropriate payloads in microfluidic flows and devices. The units could be the seat of inner or surface reactions, internal heat generation, phase transformations or the like that provoke interfacial tension inhomogeneity and eventually controlled, directed or self-propelled motion (Marangoni effect).

Bioethanol is produced through fermentation processes at very low concentrations. In order to be used as fuel in gasoline mixtures, it is necessary to have a purity of at least 99 wt.%. Both, separation and purification processes used are highly energy demanding, due to the presence of the azeotrope in the bioethanol - water mixture. A novel proposal is reactive dehydration with epoxides; however, without control the operation of the process cannot be guaranteed. Given this fact, this research presents, via dynamic simulation in Aspen Dynamics ® V8.8, the control systems for four reactive dehydration processes of bioethanol with ethylene oxide and propylene oxide, which support the operation of those technologies. It was found that it is possible to control the purity of bioethanol in reactive distillation by using temperature measures instead of compositions, thus reducing the dead times and allowing the control to be realizable.

This work addresses the formation and the internal morphology of polyelectrolyte layers obtained by the layer-by-layer method. A multimodal characterization showed the absence of stratification of the films formed by the alternate deposition of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate). Indeed the final organization might be regarded as three-dimensional solid-supported inter-polyelectrolyte films. The growth mechanism of the multilayers, followed using a quartz crystal microbalance, evidences two different growth trends, which show a dependency on the ionic strength due to its influence onto the polymer conformation. The hydration state does not modify the multilayer growth, but it contributes to the total adsorbed mass of the film. The water associated with the polyelectrolyte films leads to their swelling and plastification. The use of X-ray photoelectron spectroscopy has allowed for deeper insights on the internal structure and composition of the polyelectrolyte multilayers.

The fabrication of polyelectrolyte multilayer films (PEMs) using the Layer-by-Layer (LbL) method is one of the most versatile approaches for manufacturing functional surfaces. This is the result of the possibility to control the assembly process of the LbL films almost at will, by changing the nature of the assembled materials (building blocks), the assembly conditions (pH, ionic strength, temperature, etc.) or even by changing some other operational parameters which may impact in the structure and physico-chemical properties of the obtained multi-layered films. Therefore, the understanding of the impact of the above mentioned parameters on the assembly process of LbL materials plays a critical role in the potential use of the LbL method for the fabrication of new functional materials with technological interest. This review tries to provide a broad physico-chemical perspective to the study of the fabrication process of PEMs by the LbL method, which allows one to take advantage of the many possibilities offered for this approach on the fabrication of new functional nanomaterials.

Self-Consistent Mean-Field Calculations (SCF) have provided a semi-quantitative description of the physico-chemical behavior of six different polyelectrolyte-surfactant mixtures. The SCF calculations performed showed that both the formation of polymer-surfactant in bulk and the adsorption of the formed complexes onto negatively-charged surfaces are strongly affected by the specific nature of the considered systems, with the polymer-surfactant interactions playing a central role in the self-assembly of the complexes that, in turn, affects their adsorption onto interfaces and surfaces. This work evidences that SCF calculations are a valuable tool for deepening on the understanding of the complex physico-chemical behavior of polyelectrolyte-surfactant mixtures. However, it is worth noting that the framework obtained on the basis of an SCF approach considered an equilibrium situation which may, in some cases, be far from the real situation appearing in polyelectrolyte-surfactant systems.

Essential oils are highly volatile and non-water soluble natural products which have shown high efficiency against several insect pests. However, their low solubility in water is an important drawback for their practical applications. This work explores the design of new nanocarriers of eugenol based on silica nanoparticles capped with Pluronic F-127, a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide). Dynamic Light Scattering (DLS) and ζ-potential measurements have shown that particles capped with Pluronic F-127 are able to disperse eugenol between the polymer chains, probably due to the presence of poly(propylene oxide) blocks. Thus, nanocarriers with a structure similar to Pluronic F-127 emulsions supported on silica nanoparticles are formed. These nanocarriers are only stabilized in aqueous medium under conditions in which the ratio between the weight fractions of Pluronic F-127 and eugenol is above 1.5, otherwise phase separation appears within the first 48 h after their preparation. Furthermore, the weight fraction of silica nanoparticles is limited to low values. This work offers new possibilities for designing new aqueous based formulations with application in pests control. The use of aqueous formulations for this purpose is interesting because it makes easy the application process, handling and storage, reducing the hazards for environmental and human health associated with the use of toxic and volatile solvent. Fil: Sánchez Arribas, Natalia. Universidad Complutense de Madrid; España Fil: Guzmán, Eduardo. Universidad Complutense de Madrid; España Fil: Lucia, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Agrarias y Forestales; Argentina Fil: Toloza, Ariel Ceferino. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Centro de Investigación de Plagas e Insecticidas; Argentina Fil: Velarde, Manuel G.. Universidad Complutense de Madrid; España Fil: Ortega, Francisco. Universidad Complutense de Madrid; España Fil: Rubio, Ramón G.. Universidad Complutense de Madrid; España

Over the last two decades, understanding of the attachment of colloids to fluid interfaces has attracted the interest of researchers from different fields. This is explained by considering the ubiquity of colloidal and interfacial systems in nature and technology. However, to date, the control and tuning of the assembly of colloids at fluid interfaces remain a challenge. This review discusses some of the most fundamental aspects governing the organization of colloidal objects at fluid interfaces, paying special attention to spherical particles. This requires a description of different physicochemical aspects, from the driving force involved in the assembly to its thermodynamic description, and from the interactions involved in the assembly to the dynamics and rheological behavior of particle-laden interfaces.

Particle-laden interfaces are ubiquitous nowadays. The understanding of their properties and structure is essential for solving different problems of technological and industrial relevance; e.g. stabilization of foams, emulsions and thin films. These rely on the response of the interface to mechanical perturbations. The complex mechanical response appearing in particle-laden interfaces requires deepening on the understanding of physico-chemical mechanisms underlying the assembly of particles at interface which plays a central role in the distribution of particles at the interface, and in the complex interfacial dynamics appearing in these systems. Therefore, the study of particle-laden interfaces deserves attention to provide a comprehensive explanation on the complex relaxation mechanisms involved in the stabilization of fluid interfaces. Graphical abstract: [Figure not available: see fulltext.].

By combining controlled experiments on single interfaces with measurements on solitary bubbles and liquid foams, we show that poly( N-isopropylacrylamide) (PNIPAM) microgels assembled at air/water interfaces exhibit a solid to liquid transition changing the temperature, and that this is associated with the change in the interfacial microstructure of the PNIPAM particles around their volume phase transition temperature. We show that the solid behaves as a soft 2D colloidal glass, and that the existence of this solid/liquid transition offers an ideal platform to tune the permeability of air bubbles covered by PNIPAM and to control macroscopic foam properties such as drainage, stability, and foamability. PNIPAM particles on fluid interfaces allow new tunable materials, for example foam structures with variable mechanical properties upon small temperature changes.

The adsorption of concentrated poly(diallyldimethylammonium chloride) (PDADMAC)-sodium lauryl ether sulfate (SLES) mixtures at the water/vapor interface has been studied by different surface tension techniques and dilational viscoelasticity measurements. This work tries to shed light on the way in which the formation of polyelectrolyte-surfactant complexes in the bulk affects the interfacial properties of mixtures formed by a polycation and an oppositely charged surfactant. The results are discussed in terms of a two-step adsorption-equilibration of PDADMAC-SLES complexes at the interface, with the initial stages involving the diffusion of kinetically trapped aggregates formed in the bulk to the interface followed by the dissociation and spreading of such aggregates at the interface. This latter process becomes the main contribution to the surface tension decrease. This work aids our understanding of the most fundamental basis of the physicochemical behavior of concentrated polyelectrolyte-surfactant mixtures which present complex bulk and interfacial interactions with interest in both basic and applied sciences.

Polyelectrolyte-surfactant mixtures and their interactions with fluid interfaces are an important research field due to their use in technological applications. Most of the existing knowledge on these systems is based on models in which the polyelectrolyte concentration is around 50 times lower than that used in commercial formulations. The present work marks a step to close the gap on the understanding of their behavior under more practically-relevant conditions. The adsorption of concentrated mixtures of poly(diallyldimethyl-ammonium) chloride and sodium N-lauroyl-N-methyltaurate at the water/vapor interface with a crude mixing protocol has been studied by different surface tension techniques, Brewster angle microscopy, neutron reflectometry, and several bulk characterization techniques. Kinetically-trapped aggregates formed during mixing influence the interfacial morphology of mixtures produced in the equilibrium one-phase region, yet fluctuations in the surface tension isotherm result depending on the tensiometric technique applied. At low bulk surfactant concentrations, the free surfactant concentration is very low, and the interfacial composition matches the trend of the bulk complexes, which is a behavior that has not been observed in studies on more dilute mixtures. Nevertheless, a transition to synergistic co-adsorption of complexes and free surfactant is observed at the higher bulk surfactant concentrations studied. This transition appears to be a special feature of these more concentrated mixtures, which deserves attention in future studies of systems with additional components.

In this work, monodisperse agarose gel nanoparticles were prepared using a W/O microemulsion as a template to control the size of the obtained particles. The combination of this template method with a temperature-induced gelling and a solvent exchange methodology has allowed preparing stable aqueous dispersions of monodisperse agarose gel nanoparticles in water. The average size, measured as an apparent hydrodynamic diameter, of the obtained particles was around 150 nm. The ability of the obtained hydrogel particles for the encapsulation and release of a synthetic insecticide (azamethiphos) was tested. The results evidence that the insecticide molecules encapsulated in the fabricated nanoparticles are released following a diffusion-controlled mechanism. These results combined with the biodegradability of the agarose provide the bases for the design of a new vector with application in the control of parasites in water reservoirs. Fil: Argudo, Pablo G.. Universidad Complutense de Madrid. Facultad de Ciencias Químicas; España Fil: Guzmán, Eduardo. Universidad Complutense de Madrid. Facultad de Ciencias Químicas; España Fil: Lucia, Alejandro. Universidad Nacional de Luján. Instituto de Ecología y Desarrollo Sustentable. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ecología y Desarrollo Sustentable; Argentina Fil: Rubio, Ramón G.. Universidad Complutense de Madrid. Facultad de Ciencias Químicas; España Fil: Ortega, Francisco. Universidad Complutense de Madrid. Facultad de Ciencias Químicas; España

Soft assemblies obtained following the Layer-by-Layer (LbL) approach are accounted among the most interesting systems for designing biomaterials and drug delivery platforms. This is due to the extraordinary versatility and flexibility offered by the LbL method, allowing for the fabrication of supramolecular multifunctional materials using a wide range of building blocks through different types of interactions (electrostatic, hydrogen bonds, acid-base or coordination interactions, or even covalent bonds). This provides the bases for the building of materials with different sizes, shapes, compositions and morphologies, gathering important possibilities for tuning and controlling the physico-chemical properties of the assembled materials with precision in the nanometer scale, and consequently creating important perspective for the application of these multifunctional materials as cargo systems in many areas of technological interest. This review studies different physico - chemical aspects associated with the assembly of supramolecular materials by the LbL method, paying special attention to the description of these aspects playing a central role in the application of these materials as cargo platforms for encapsulation and release of active compounds.

The effect of the incorporation of hydrophilic titanium dioxide (TiO2) nanoparticles on the interfacial properties of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) has been evaluated combining interfacial thermodynamic studies, dilatational rheology, and Brewster angle microscopy (BAM). The results show that the TiO2 nanoparticles are able to penetrate DPPC layers, modifying the organization of the molecules and, consequently, the phase behavior and viscoelastic properties of the systems. Measurements of dilational viscoelasticity against the frequency have been performed, using the oscillatory barrier method, at different values of the surface pressure corresponding to different degrees of compression of the monolayer. The presence of TiO2 nanoparticles also affects the dynamic response of the monolayer modifying both the quasi-equilibrium dilatational elasticity and the high frequency limit of the viscoelastic modulus. The principal aim of this work is to understand the fundamental physicochemical bases related to the incorporation of specific nanoparticles of technological interest into the interfacial layer with biological relevance such as phospholipid layers. This can provide information on potential adverse effects of nanoparticles for health and the environment.

The contact angle of particles attached to fluid interfaces plays a key role in many scientific and technological aspects of particle-laden layers. In spite of the recognized importance, the laws that govern this property are still poorly understood. The main problem associated with the study of this property is that multiple variables are involved in the wetting process of particles by fluid interfaces. Such variables are associated with the chemical nature of both the particles and the fluid phases, and with the particle's size. Understanding of the different aspects controlling the contact angle of particles is a physico-chemical challenge, and is very important because of the many technological aspects in which particle laden interfaces are involved. This review discusses the current status and the aspects to be dealt with in the near future in the study of the contact angle of particles attached to fluid interfaces. (C) 2014 Elsevier Ltd. All rights reserved.

Lithium cations are known to form complexes with the oxygen atoms of poly(oxyethylene) chains. The effect of Li + on the surface properties of three block-copolymers containing poly(oxyethylene) (PEO) have been studied. Two types of copolymers have been studied, a water soluble one of the pluronic family, PEO–b-PPO–b-PEO, PPO being poly(propyleneoxyde), and two water insoluble ones: PEO–b-PS and PEO–b-PS–b-PEO, PS being polystyrene. In the case of the pluronic the adsorption kinetics, the equilibrium surface tension isotherm and the aqueous/air surface rheology have been measured, while for the two insoluble copolymers only the surface pressure and the surface rheology have been studied. In all the cases two different Li + concentrations have been used. As in the absence of lithium ions, the adsorption kinetics of pluronic solutions shows two processes, and becomes faster as [Li + ] increases. The kinetics is not diffusion controlled. For a given pluronic concentration the equilibrium surface pressure increases with [Li + ], and the isotherms show two surface phase transitions, though less marked than for [Li + ] = 0. A similar behavior was found for the equilibrium isotherms of PEO–b-PS and PEO–b-PS–b-PEO. The surface elasticity of these two copolymers was found to increase with [Li + ] over the whole surface concentration and frequency ranges studied. A smaller effect was found in the case of the pluronic solutions. The results of the pluronic solutions were modeled using a recent theory that takes into account that the molecules can be adsorbed at the surface in two different states. The theory gives a good fit for the adsorption kinetics and a reasonably good prediction of the equilibrium isotherms for low and intermediate concentrations of pluronic. However, the theory is not able to reproduce the isotherm for [Li + ] = 0. Only a semi-quantitative prediction of the surface elasticity is obtained for [pluronic] ⩽ 1 × 10 −3 mM.

The effect of silica nanoparticles on the thermodynamic and structural properties of monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), palmitic acid (PA) and a mixture of them, has been investigated using a combination of Langmuir trough technique and Brewster Angle Microscopy (BAM). This study shows that the presence of nanoparticles in the aqueous sub-phase affects the surface pressure-area (Pi-A) isotherm, leading to a different phase behavior of the monolayer. The observed effects are explained assuming the incorporation of nanoparticles into the monolayer driven by the adsorption of lipid molecules onto the silica which makes the nanoparticles more hydrophobic. This process hinders the ordering and affects the composition of the investigated lipid monolayers significantly modifying also their quasi-equilibrium dilational elasticity. The reported results are useful in the framework of a wider study aimed at elucidating the impact of nanoparticles on the physico-chemical properties of pulmonary surfactant, being DPPC its major component and PA utilized as an important component in the formulation of therapeutic substitutes. (C) 2011 Elsevier B.V. All rights reserved.

The hydrophobicity of a particle surface can be tuned by the addition of surfactants that change the surface free energy for their attachment to a liquid interface. In this work, we report an experimental study where the wettability properties of silica nanoparticles are modified by the adsorption of alkyltrimethylammonium surfactants (CnTAB, n = 12, 16) on the surface of the particles. We have pointed out that the wettability of the complexes is controlled by an intricate balance of electrostatic and hydrophobic interactions between the particle surface and the surfactant. These interactions play an important role in the structure of the surfactant-particle nanocomposite interfacial layer.

The adsorption of polyelectrolytes onto negatively charged substrates has been studied using dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The polymers studied belong to two families of cationic polymers frequently used as hair conditioners. One of the families is formed by four polymers derived from the poly(diallyl-dimethylammonium chloride), and the other one is formed by two polysaccharide polymers. The control parameters in the study of both families of polymers were the charge density and the rigidity of the chains. The adsorption kinetics of the polymers showed a complex behavior with several processes involved. The total adsorbed amount depends on the nature of the polymer chains, on the ionic strength, and on polymer concentration of the solutions. The comparison of the mass of the adsorbed layers obtained from D-QCM and from ellipsometry allowed us to calculate the water content of the layers. This variable takes high values for all the polymers studied, which is related to the non-homogeneous character of the adsorbed layer, as confirmed by AFM. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be in the MPa range as for rubber-like polymer system. The adsorptions of the polyelectrolytes are not driven exclusively by the electrostatic interactions. entropic contributions being of fundamental importance, and arise mainly from the release of counterions during the adsorption process, and/or the number of loops and tails of the adsorbed chains protruding into the solution. (C) 2010 Elsevier B.V. All rights reserved.

Multilayers of sodium salt of poly(4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium) chloride (PDADMAC) have been built layer by layer (LbL) both at the solid/aqueous interface (solid supported) and the air/aqueous interface (liquid supported). For the solid-supported multilayers, the adsorption kinetics and the complex shear modulus were measured using a dissipative quartz crystal microbalance and a null ellipsometer. A bubble tensiometer was used to measure the adsorption kinetics and the elasticity modulus of the liquid-supported multilayers. At the solid/aqueous interface, adsorption kinetics changes with the number of adsorbed layers. However, at the air/aqueous interface, PSS dynamics were the same for all adsorbed layers except the first. Conversely, the adsorption kinetics of PDADMAC at the air/water surface differed between those layers close to the interface and those far from it. Multilayers grow at the air/water interface by an intrinsic-charge-compensation process, whereas, for the same ionic strengths, solid-supported layers deposit by the extrinsic-charge-compensation process. No significant differences were found between the recoverable dilational storage modulus of the liquid-supported multilayers and the real part of the shear modulus of the solid-supported ones built at the same ionic strength. The values of the modulus are in the MPa range, which corresponds to gel-like films. This result is in agreement with the strong hydration degree of the LbL films calculated from ellipsometry measurements.

An experimental study is here presented on the properties of aqueous dispersions containing carbon nanoparticles and different ionic surfactants which can modify the degree of hydrophobicity/philicity of particles favoring their transfer from the dispersion bulk to the interfacial layer. Aim of this work is to understand the particle-surfactant and particle-fluid interface interactions and their effect on those macroscopic surface properties of the mixed systems which are expected related to the stability and structure of the respective particle stabilized foams. To this purpose a systematic characterization of dispersions have been carried out, based on surface tension measurement against the surfactant concentration, using a drop Profile Analysis Tensiometer (PAT). These results have been crossed with the characterization of the bulk dispersion by Dynamic Light Scattering (DLS) and ζ-potential measurements to check the effects of surfactant on the particle aggregation and on the particle surface charge, respectively. The stability of the foams obtained with the same compositions has been also investigated and correlated to the other surface and bulk properties.

The interest of polymer-surfactant systems has undergone a spectacular development in the last thirty years due to their complex behavior and their importance in different industrial sectors. The importance can be mainly associated with the rich phase behavior of these mixtures that confers a wide range of physico-chemical properties to the complexes formed by polymers and surfactants, both in bulk and at the interfaces. This latter aspect is especially relevant because of the use of their mixture for the stabilization of dispersed systems such as foams and emulsions, with an increasing interest in several fields such as cosmetic, food science or fabrication of controlled drug delivery structures. This review presents a comprehensive analysis of different aspects related to the phase behavior of these mixtures and their intriguing behavior after adsorption at the liquid/air interface. A discussion of some physical properties of the bulk is also included. The discussion clearly points out that much more work is needed for obtaining the necessary insights for designing polymer-surfactant mixtures for specific applications.

We report here a review of particle-laden interfaces. We discuss the importance of the particle’s wettability, accounted for by the definition of a contact angle, on the attachment of particles to the fluid interface and how the contact angle is strongly affected by several physicochemical parameters. The different mechanisms of interfacial assembly are also addressed, being the adsorption and spreading the most widely used processes leading to the well-known adsorbed and spread layers, respectively. The different steps involved in the adsorption of the particles and the particle-surfactant mixtures from bulk to the interface are also discussed. We also include here the different equations of state provided so far to explain the interfacial behavior of the nanoparticles. Finally, we discuss the mechanical properties of the interfacial particle layers via dilatational and shear rheology. We emphasize along that section the importance of the shear rheology to know the intrinsic morphology of such particulate system and to understand how the flow-field-dependent evolution of the interfacial morphology might eventually affect some properties of materials such as foams and emulsions. We dedicated the last section to explaining the importance of the particulate interfacial systems in the stabilization of foams and emulsions.

Highly water repellent coatings known as superhydrophobic (SH) are related to surfaces with contact angles above 150° and a very small hysteresis. Among many practical needs to which this property can be devoted, a very interesting field where the SH coating technology can be employed is represented by the protection of hulls in seawater environments from the biofouling colonization. The absence of toxic materials in the final product and in the preparation stage holds to preferable bio-compatible coatings, basing the action on low surface energy exerting physical prevention of bioadhesion of colonizing organisms. In this work we report on field tests of a superhydrophobic ‘green’ coating on aluminum and glass substrates in real seawater mesocosms conditioned/not conditioned by the natural photoperiod and the photobiology development. In particular, the performance, durability and fouling removability of the here tested SH coating is evaluated with immersions focusing on two time scales (days and weeks), emphasizing the role of conditioning film and initial biological slimes formation. Implications of results here obtained on ship hulls protection and indications for future investigations to be held are discussed.

The combination of interfacial thermodynamic, dilational rheology, and Brewster angle microscopy (BAM) has allowed the evaluation of the effect of hydrophobic fumed silica nanoparticles on the interfacial properties of Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC). Fumed silica particles modify the surface pressure-area (II-A) isotherm of DPPC, modifying both the phase behavior and the collapse conditions. These modifications are strongly dependent on the relative quantity of DPPC and Nanoparticles initially spread at the air-water interface. The incorporation of nanoparticles at the fluid interface alters the balance of interactions within the monolayer, thus leading to the disruption of the interfacial structure and consequently of the lipid packing. Measurements of dilational viscoelastic modulus against the frequency have been carried out by the mean of the oscillatory barrier method at different degrees of compression of the monolayer. The dynamic response is also modified by the presence of increasingly amount of nanoparticles at the interface. DPPC, being a lipid generally used as classical model for the study of biological relevant systems, the results here obtained can be used to deepen on the understanding of the effects of nanoparticles on the interfacial properties of biosystems.

The use of polymer and polymer - surfactant mixtures for designing and developing textile and personal care cosmetic formulations is associated with various physico-chemical aspects, e.g. detergency and conditioning in the case of hair or wool, that determine their correct performances in preserving and improving the appearance and properties of the surface where they are applied. In this work, special attention is paid to the systems combining polycations and negatively charged surfactants. The paper introduces the hair surface and presents a comprehensive review of the adsorption properties of these systems at solid-water interfaces mimicking the negative charge and surface energy of hair. These model surfaces include mixtures of thiols that confer various charge densities to the surface. The kinetics and factors that govern the adsorption are discussed from the angle of those used in shampoos and conditioners developed by the cosmetic industry. Finally, systems able to adsorb onto negatively charged surfaces regardless of the anionic character are presented, opening new ways of depositing conditioning polymers onto keratin substrates such as hair.

We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.

The interaction between palmitic acid (PA) and silica nanoparticles at the water/hexane has been studied in relation to the ability in the stabilization of water in oil emulsions. To this aim, emulsions formed by aqueous silica nanoparticle dispersions and PA solutions in hexane have been investigated. In contrast with other previous studies, where the interaction of nanoparticles and surfactant molecules occurs already in the bulk phases, here PA and silica nanoparticles are dissolved/dispersed in different phases and interact only at the liquid–liquid interface. As for other particle-stabilized emulsions, the investigated system can provide emulsions stable over very long time because of the formation of nanoparticle–surfactant complexes, which accumulate at the droplets interface, providing a steric hindrance of coalescence. However, the present study has pointed out that these enhanced stability conditions are achieved only when the amount of surfactant leads to the formation of single surfactant layers onto the particles surface. Whereas an excess surfactant amount interacts with the particles, the formation of multilayers would be possible, and the formed complexes can be redispersed again in the aqueous bulk phase. This recalls to a scenario where the degree of hydrophobization of the particles is critical for the emulsion stabilization. This can be easily tuned by the control of the surfactant concentration and the volume ratio between the liquid phases. Interfacial tension measurement of the same couple of liquids, has allowed us to explore in more detail such phenomena, revealing a non-monotonic evolution of the dynamic interfacial tension at large PA concentrations. Also these observations can be rationalized assuming the redispersion of the complexes mentioned above, which allows explaining the interfacial tension evolution on the basis of the succession of adsorption and desorption of the different species – nanoparticles, nanoparticle–PA complexes, PA molecules – involved.

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail. Fil: Mendoza, Alma J.. Universidad Complutense de Madrid. Facultad de Ciencias Químicas; España Fil: Guzmán, Eduardo. Consiglio Nazionale delle Ricerche; Italia Fil: Martínez Pedrero, Fernando. Universidad Complutense de Madrid; España Fil: Ritacco, Hernán Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Rubio, Ramón G.. Universidad Complutense de Madrid; España Fil: Ortega, Francisco. Universidad Complutense de Madrid; España Fil: Starov, Victor M.. University of Loughborough; Reino Unido Fil: Miller, Reinhard. Max Planck Institute for Colloids and Interfaces; Alemania

In recent years, polymer multilayers built by electrostatic self-assembly (LbL) have been frequently used in the fabrication of functional materials. They allow building of supramolecular structures loaded with active compounds which can be delivered to their environment under specific conditions. The growing interest in the building and processing of LbL multilayers, in a wide range of applications, is the result of the simplicity and extraordinary versatility of these films. This chapter reviews the current state and trends in the study of polyelectrolyte assemblies and their applications, mainly those related to drug storage and delivery. These applications are closely correlated to the building process and properties of the films, especially internal structure and mechanical properties. As a consequence it is essential to control the different aspects that can modify the functionality of the films. In addition, for specific applications, different morphological requirements must be considered, which can be easily achieved by the LbL method that allows the fabrication of supramolecular architectures from flat films to capsules with different geometries and sizes These complex objects can be loaded with drug molecules, and can be functionalized to make them sensitive to external stimuli. In this way, it is possible to obtain a controlled delivery of the drug at the desired organs. Moreover, it is possible to use different administration routes. In conclusion, the use of polyelectrolyte multilayers has opened an extraordinary variety of possibilities in the field of manufacturing of drug delivery platforms.

An experimental study focused on the problem of tailoring of solid foams with controlled structural and chemical features produced by drying and/or sintering of liquid foams stabilized by nanosized materials is presented here.The aim of this study is a better understanding of the relation between the interfacial properties of mixed systems containing nanoparticles (NP) at different degree of hydrophobicity and surfactants and the stability/structure of the corresponding liquid and solid foams.The interfacial properties of the single particle laden interfacial layers were characterized by dynamic interfacial tension and interfacial rheology measurements as well as the stability of the corresponding particle stabilized foams. The solid foams obtained from these systems, according to specifically developed procedure, were then analyzed by SEM techniques as regards their morphology (size porosity, cell structure) and surface chemical composition. © 2013 Elsevier B.V.

We report an experimental study on the growth and adsorption kinetics of polyelectrolyte multilayers (PEMs). PEMs composed of poly(diallyldimethyl ammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS), and of poly(allylamine hydrochloride) (PAH) and PSS polyelectrolyte pairs were built and studied via dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results have pointed out that the growth trend of PEMs may be controlled by the assembly conditions that modify the layer structure and subsequently the film features. The study of adsorption kinetics of the layers suggests that, even though interdiffusion may take place during the growth of PEMs, it does not determine the growth mechanism. Analysis of the mechanical properties allows confirmation of the scenario proposed for the explanation of the PEMs growth. Independent of growth type, the adsorption kinetics of the layers is a bimodal process. The results here presented allow us to rule out any correlation between growth mechanism and adsorption dynamics. Fil: Guzmán, Eduardo. Universidad Complutense de Madrid; España Fil: Ritacco, Hernán Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Ortega, Francisco. Universidad Complutense de Madrid; España Fil: Rubio, Ramón G.. Universidad Complutense de Madrid; España

The effect of silica nanoparticles on the dynamic behavior of monolayers composed by 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with Palmitic acid (PA), has been investigated by comparing the dilational rheological response of these lipid layers, spread on water and on a silica nanoparticle dispersion. To this aim, the dilational viscoelasticity has been measured against the frequency of the surface area perturbation of the monolayer, according to the Oscillatory Barrier method in a Langmuir trough. These measurements were performed at different values of the surface pressure, corresponding to different degrees of compression of the monolayer. The results show that the incorporation of particles in the layer induces additional surface kinetic processes and, depending on the surface pressure, modifies both the quasi-equilibrium dilational elasticity and the high frequency limit of the viscoelastic modulus. Another important effect concerns the linearity of the dilational rheological response which is appreciably worsened by the presence of nanoparticles. With DPPC being the major component of pulmonary surfactant and PA used as a component in synthetic substitutes of it, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.

This work is focused on the physico-chemical characterization of carbonaceous nanometric particulate produced from controlled combustion sources and on its effects on dynamic interfacial tension and surface dilational rheology of aqueous interfaces and on foamability. Since large quantities of surfactants are introduced into waste waters and water bodies by industrial activities, the properties of carbon particulate-laden liquid interfaces have been investigated in connection with the presence of surfactants. Aim of this work is to evidence the occurrence of synergetic effects between carbonaceous particulate and surfactants in affecting the above properties. Two carbonaceous particulates have been selected as representative of aromatic and aliphatic soot. These particulates have been sampled from benzene and ethylene premixed laminar flames burning in fuel-rich conditions and characterized in terms of micro and nanostructures and size distribution. Water-air (W/A) interfacial properties of dispersions of the above particulate have been investigated in presence of hexadecyltrimethylammonium bromide (CTAB), a widely used cationic surfactant. The dynamic surface tension and the surface dilational rheological behaviour have been studied in relationship with foam stability for environmental purpose and, as perspective, to explore new techniques involving the set up of stable dispersion of carbon particulate for the synthesis of microporous materials to be employed in CO2 capture. Comparison with the results previously obtained with commercially available carbon black particles are also given. (C) 2012 Elsevier B.V. All rights reserved.

The properties and structure of different types of interfacial layers obtained from aqueous dispersions of nanometric silica and palmitic acid (PA) have been studied and characterized by different diagnostics and measurements. The investigations concern PA monolayers spread on the silica dispersions, dispersions in contact with PA solutions in oil and silica dispersions containing PA, aiming at elucidating the role of the PA interaction with the particles and investigating the surface-activity of the originated silica-PA complexes. Drop shape tensiometry was utilized to measure the dynamic surface and interfacial tension while a Langmuir trough apparatus was used to obtain compression isotherms of the spread PA layers and to measure the dilational viscoelasticity according to the oscillating barrier method. Brewster angle microscopy and ellipsometry were utilized to investigate the lateral and vertical structure of the interfacial layers. From this multifold approach emerges a complex picture of the features of these interfacial layers that can be rationalized on the basis of the adsorption of PA on the particle surface. The results evidence a threshold in PA adsorption above which particles change from hydrophilic to partially hydrophobic, promoting their incorporation into the interfacial layer.

X-ray reflectivity and adsorption kinetics of two different polyelectrolyte multilayers have been studied for polyelectrolyte multilayers (PEMs) composed of poly(diallyldimethyl ammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) polyelectrolyte pair and of poly(allylamine hydrochloride) (PAH) and PSS. The two characteristic times that describe the adsorption kinetics were found to be related to the X-ray reflectivity results, and their dependence on the number of adsorbed layers is depends on the growth mechanisms (linear or superlinear) of the films. Therefore, it has been concluded that there is a correlation between the adsorption kinetics and the internal structure of the films. The time allowed for the adsorption of each polyelectrolyte controls the extension of the interdiffusion within the multilayer, and therefore whether the film is formed by stratified layers or it has a uniform structure except for the first and last layers. While the roughness of the (PDADMAC + PSS)n films strongly depends on the adsorption time, it is almost independent of it for the (PAH + PSS)n multilayer. This behavior correlates with that of the longest characteristic time of the adsorption kinetics. Fil: Guzmán, Eduardo. Universidad Complutense de Madrid; España Fil: Ritacco, Hernán Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Sur; Argentina Fil: Ortega, Francisco. Universidad Complutense de Madrid; España Fil: Rubio, Ramón G.. Universidad Complutense de Madrid; España